Information flow within the nervous system requires the perpetuation of ionic gradients along neurons. In many neurons, effective and efficient perpetuation of such gradients along axons requires electrical insulation. Myelin is a specialized lipid-rich, dielectric substance that ensheathes neuronal axons that serves this insulating function, promoting efficient nerve impulse transmission (Morell and Quarles (1999) Basic Neurochemistry: molecular, cellular, and medical aspects. In Siegel G J, ed. Myelin Formation, Structure, and Biochemistry. Lippincott-Raven Publishers, 79-83). The nervous system contains high levels of myelin, which is especially enriched where many myelinated axons are bundled together, such as in tracts of the spinal cord and spinal nerve roots, nerves in the peripheral nervous system, and fiber tracts in the brain, collectively called “white matter” (as opposed to “grey matter”). Because non-nervous system tissue lacks myelin, the presence of myelin can distinguish peripheral nerve tissue from other tissue types, the spinal cord and spinal nerve roots from non-nervous elements of the vertebral column, and white matter from grey matter.
Due to its important biological functions in the nervous system and its vulnerability in disease, several techniques have been developed to visualize and characterize myelin histopathology. These can be broadly divided into those based upon antibody immunohistochemistry (IHC) (Horton and Hocking (1997) Cereb. Cortex 7:166-177) and more traditional histochemical procedures. The classic histochemical stains include luxol fast blue MBN (Kluver and Barrera (1953) J Neurosci Methods 153: 135-146; Presnell and Schreibman (1997) Humanson's Animal Tissue Techniques, 5th ed.; Kiernan (1999) Histological and Histochemical Methods: Theory and practice, 3rd ed.; Bancroft and Gamble (2002), Theory and Practice of Histological Techniques, 5 ed. and Sudan Black B (Lison and Dagnelie (1935) Bull. d'Histologie Appliquee 12: 85-91). Traditional chromogenic methods also include the Palweigert method ((Weigert (1884) Fortschr Deutsch Med 2: 190-192, (1885) Fortschr Deutsch Med 3:236-239; Clark and Ward (1934) Stain Technol 54:13-16), the Weil stain (Weil (1928) Arch Neurol Psychiatry 20:392-393; Berube et al. (1965) Stain Technol 40:53-62)), the Loyez method (Cook (1974) Manual of Histological Demonstration Methods, 5th ed.), and a method based on horse serum followed by subsequent reaction with diaminobenzidine (McNally and Peters (1998) J Histochem Cytochem 46:541-545). In addition, modified silver stains including the Gallyas method (Pistorio et al. (2005) J Neurosci Methods 153: 135-146) and Schmued's gold chloride technique (Schmued and Slikker (1999) Brain Res 837:289-297) have also been used as simple, high- resolution histochemical markers of myelin. More recently, fluoromyelin (Kanaan et al. (2005) Am J Physiol Regul Integr Comp Physiol 290:R1105-1114) and NIM (Xiang et al. (2005) J Histochem Cytochem 53:1511-1516) were introduced as novel myelin dyes, which enable quick and selective labeling of myelin including brain tissue sections. In addition, fluorescent stilbenzene derivative have been shown to selectively bind to myelin in an animal's brain tissue (Wu et al., (2006) J Histochem Cytochem 54: 997-1004).
However, the lack of in vivo molecular probes has limited the progress of peripheral nervous system tissue myelin imaging and hindered efficacy evaluation of novel peripheral nervous system tissue myelin repair therapies during their development.